Question : How Do I Track Network Traffic From selective PCs to its Destination

Guys,

I need a good way of tracking specific network traffic from selective PCs  on my network to its destination. To ascertain whether anything strange is happening to the traffic in between the client & the destination. Is there a tool you could recommend that is easy to use and understand?
The destination host is on the other side of a WAN Link. From my firewall to the destination via the WAN Link, i can ask BT to perform monitoring.

Answer : How Do I Track Network Traffic From selective PCs to its Destination

I prefer NTOP for longer-term monitoring and history
http://www.openxtra.co.uk/freestuff/ntop-xtra.php
This will show you all the inside hosts and all the outside hosts, what protocols used, and and how much data per individual host.
Just make sure that the machine running NTOP is connected to a mirror/span port on the same switch that connects the firewall to the LAN.